The present invention relates to a thermal printhead including a plurality of heating elements and a plurality of drive IC""s which control operation of these heating elements. The present invention also relates to a printer incorporating such a thermal printhead.
In recent years, portable printers and copiers are becoming popular. These portable apparatuses are powered by batteries, and include a thermal printhead for making a print on a predetermined recording paper.
Generally, the thermal printhead includes a plurality of heating elements and a plurality of drive IC""s for controlling these heating elements. In order to print, the heating elements must be supplied with a head voltage, whereas the drive IC""s must be supplied with a logic voltage. In the portable apparatus, the head voltage and the logic voltage are supplied by the batteries incorporated in the apparatus.
Therefore, in the portable apparatus, it is required that power consumption be small, and a drive voltage be low. Further, consideration must be paid to a decrease in power voltage during use.
Given these requirements, in a conventional thermal printhead, improvement has been made to the construction of the heating elements in order to reduce power consumption and to enable use at a low head voltage.
However, according to the conventional thermal printhead, the logic voltage is fixed to 3.3 volts or 5 volts. For this reason, at least two kinds of the thermal printhead have to be designed and manufactured, resulting in a problem of increased manufacturing cost. Another problem is that in order to avoid reduction in the logic voltage due to use over a period of time, the supply of the logic voltage from the batteries is made via a DCxe2x80x94DC converter, resulting in a problem of increased parts cost and assembling cost.
The present invention is made under these circumstances, and it is an object of the present invention to provide, without increasing the cost of manufacture as long as possible, a thermal printhead capable of operating at any power voltage in a range assumed for the battery power.
Another object of the present invention is to provide a portable printer incorporating such a thermal printhead.
In order to achieve the objects, the present invention makes use of the following technical means.
A thermal printhead provided by a first aspect of the present invention comprises a plurality of heating elements heated by a head voltage for printing on a recording paper, and a plurality of drive IC""s powered by a logic voltage for driving the heating elements. The printing can be performed at whatever value of the head voltage within a range from 2.7 volts through 8.5 volts.
The printing paper may be a thermal paper. If the thermal paper is not used, an ink ribbon may be used.
According to a preferred embodiment of the present invention, the drive IC""s operate at whatever value of the logic voltage within a range from 2.7 volts through 5.5 volts.
Preferably, the head voltage and the logic voltage can be set independently of each other.
Preferably, the thermal printhead according to the present invention further comprises voltage varying means for varying a pulse width of the head voltage during the printing in response to variation in the head voltage.
Preferably, each of the heating elements has an effective print length in a sub-scanning direction, a pixel to be printed per printing datum has a print length in the sub-scanning direction, and the effective print length is generally equal to an n-th of the print length. Herein, n is a natural number not smaller than 2.
Preferably, each of the drive IC""s incorporates a plurality of transistors connected to the heating elements.
Preferably, the transistors are MOS field effect transistors.
A printer provided by a second aspect of the present invention comprises power supply means and a thermal printhead. The thermal printhead includes a plurality of heating elements heated by a head voltage for printing on a recording paper, and a plurality of drive IC""s powered by a logic voltage for driving the heating elements. The printing is performed at whatever value of the head voltage within a range from 2.7 volts through 8.5 volts.
Preferably, the drive IC""s operate at whatever value of the logic voltage within a range from 2.7 volts through 5.5 volts.
Preferably, the power supply means includes a battery.
According to the present invention, as long as the head voltage supplied to the heating elements is within the range from 2.7 volts to 8.5 volts, it is possible to form an image on the printing paper. Further, as long as the logic voltage supplied to the drive IC""s is within the range from 2.7 volts to 5.5 volts, it is possible to drive the drive IC""s. Therefore, operation becomes possible at any power voltage in a range assumed for the battery power. Further, the invention eliminates the need to design and manufacture two kinds of the thermal printhead. Thus, it becomes possible to reduce the cost of manufacture including development cost.
Other characteristics and advantages of the present invention will become clearer from the following description in detail to be presented with reference to the accompanying drawings.